Thermoset resins (or thermosets) have the advantage of having a high mechanical strength and a high thermal and chemical resistance and, for this reason, can replace metals in certain applications. They have the advantage of being lighter than metals. They can also be used as matrices in composite materials, as adhesives, and as coatings. Among the thermoset polymers, mention may be made of unsaturated polyesters, phenoplasts, polyepoxides, polyurethanes and aminoplasts.
Conventional thermosetting resins must be processed; in particular, they must be molded so as to immediately obtain the shape appropriate for the final use. This is because transformation is no longer possible once the resin is polymerized, other than machining which often remains difficult. Soft or hard parts and composites based on thermosetting resins can neither be transformed nor shaped; they cannot be recycled or welded.
In parallel to thermosetting resins, a class of polymer materials, thermoplastics, has been developed. Thermoplastics can be formed at high temperature by molding or by injection-molding, but have mechanical and thermal and chemical resistance properties that are less advantageous than those of thermoset resins.
In addition, the forming of thermoplastics can only be carried out in very narrow temperature ranges. This is because, when they are heated, thermoplastics become liquids, the fluidity of which varies abruptly in the region of the melting points and glass transition temperatures, thereby making it impossible to apply to them a whole variety of transformation methods that exist for glass and for metals for example.
In this context, vitrimer resins have been designed for the purpose of allying the advantages of both thermosets and thermoplastics. These materials have both the mechanical and solvent-resistance properties of thermoset resins and the capacity to be reshaped and/or repaired of thermoplastic materials. These polymer materials which are capable of indefinitely going from a solid state to a viscoelastic liquid, like glass, have been denoted “vitrimers”. Contrary to thermoplastics, the viscosity of vitrimers varies slowly with temperature, thereby making it possible to use them for the production of objects that have specific shapes incompatible with a molding process, without using a mold or precisely controlling the forming temperature.
The specific properties of vitrimers are linked to the capacity of their network to reorganize above a certain temperature, without modifying the number of intramolecular bonds or depolymerizing, under the effect of internal exchange reactions. These reactions lead to a relaxing of the stresses within the material which becomes malleable, while preserving its integrity and remaining insoluble in any solvent. These reactions are made possible by the presence of a catalyst. In the case of vitrimers of epoxy-anhydride type, it has been suggested to use as catalyst a zinc, tin, magnesium, cobalt, calcium, titanium or zirconium metal salt, preferably zinc acetylacetonate (WO 2012/101078). Likewise, various catalysts have been suggested for use in hybrid thermoset/supramolecular systems obtained from a thermosetting resin, from a curing agent of anhydride-type or preferably of acid type and from a compound comprising an associative group and a function allowing grafting thereof onto the thermosetting resin (WO 2012/152859). These catalysts can be based on various metals, including titanium, and are in the form of various salts, in particular of alkoxides (or alcoholates) such as titanium isopropoxide, although zinc acetylacetonate is, here again, preferred.
Titanic acid esters or titanic acid polymer esters have moreover been proposed in documents FR 1419754 and GB 1069439 for efficiently accelerating the curing, by polycarboxylic anhydrides, of cycloaliphatic polyepoxides in which at least one epoxide group is in a five-membered ring.
In addition, a catalyst system of titanium aryloxy type, in particular titanium phenolate, such as titanium catecholate, has been suggested in document FR 2584412 for facilitating the anionic polymerization of epoxide resins.
As it happens, the inventors have demonstrated that the stresses developed within the materials obtained for example from zinc acetylacetonate are less completely and less rapidly relaxed than within materials prepared from catalysts in the form of specific organometallic titanium complex. The latter thus exhibit better deformation properties, which are more compatible with an industrial thermoforming process, which requires very rapid deformation and relaxing of the stresses. In addition, contrary to the materials obtained from other titanium catalysts, this ability to deform is not obtained to the detriment of the crosslinking density, and therefore of the mechanical properties of the material.
Furthermore, another drawback of zinc acetylacetonate is the fact that at the temperatures (from 250 to 350° C.) required for transformation, this catalyst is not sufficiently stable, thereby causing gas to be given off during hot manipulations of the material, resulting in a loss of mass measured in particular by thermogravimetric analysis (TGA).